Page:Cyclopaedia, Chambers - Volume 2.djvu/1059

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■tratcr mull be carried off, and the reft kept dry and piled: that fide of the river where you are to work is to be inclo- fed with coffer-dams, and the current to have its liberty on the other fide. See Foundation. Palladlo's directions are, firft, to make choice of that place in a river which has the leaft depth of water, and where the ground is even and firm, efpecially rock or gravel ftone ; fecondly, to avoid thofe places where there are voragos, or whirlpools, and •where the bottom is foft fand or gravel, in regard fuch mat- ters are eafily carried away by the violence of water, which in time alters the bed of a river, and faps the foundation of the peers; thirdly, to pitch on a ftreight part of a river, fince otherwife the turns and windings being worn away in time, the bridge is in danger of being left infulate ; befides being liable to be choaked up with the filth, and other mat- ters commonly gathered in the turns of rivers.

Bridges are either built of ftone or timber, according as there is a conveniency, or plenty of the one material or the other in the place.

5/cw-Bridges, are compofed of peers, arches, and butments, made of hewn ftone, fometimes alfo intermixed with brick ; as, the bridge of Tholoufe, the plinths whereof are of ftone, as alfo the quoins of the arches, and fome bonding courfes, and copings ; but the reft, as the arches, walls, and but- ments, of brick.

T^W^w-Bridges, called by the Latins, pontes fublicii, confift of beams and joifts fuftained by punchions, well cramped and bound together.

Sturm ius has a diflertation exprefs on the ftructure of a wooden-bridge: Difp. de Ponte Sublido. Francof. 1709.

^rn-BRiDGE, pout de jonc, is made of large fheaves of rufhes growing in marfhy grounds, which they cover over with boards or planks. They ferve for crofling ground that is boggy, miry, or rotten.

The Romans had alfo a fort of fubitaneous bridges, made by the foldiers, of boats, or fometimes of caiks, leathern bot- tles or bags, or even of bullocks bladders blown up, and fatt- ened together, called afcogefri. Pitifc. L. Ant. T. 2. p. 464. feq. voc. Pontes. Du Cange Glojf. Lat. T. I. p. 350. M. Couplet gives the ftructure of a portable bridge, 200 foot long, eafily taken afunder and put together again, and which forty men may carry.— fid. Du Hamel Hiji. Reg. Acad. Sci- en. I. .3. feci. 5. c. 4, ^.273.

Frezier fpeaks of a wonderful kind of bridge at Apurima in Lima, made of ropes, formed of the bark of a tree. — Fid. Frez. Voyag. South-Sea, p. 184.

Pendent, or Hanging, called alio Philofophical Bridges, are thofe not fupported either by pofts or pillars, but hung at large in the air, only fuftained at the two ends, or but- ments.- — Inftances of fuch bridges are given by Palladio, and others. — Vid. Vogels Modern. Bau-KunJ}, Tab. 26. feq. Wolf. L. Math. p. 277. voc. Brucke. Kirch. Mund. Subterr. I, 1. c. 3. T. 1. p. 14.

Dr. Wallis gives the defign of a timber-bridge, feventy foot long, without any pillars, which may be ufeful in fome pla- ces where pillars cannot conveniently be ere&ed. Pbilof. Tranf. N° 163. p. 714.. Dr. Plot allures us, that there was formerly a large bridge over the caftle-ditch at Tutbury in Staffordfhire, made of pieces of Timber none much above a yard long, and yet not fupported underneath, either with pillars, or arch- work, or any other fort of prop whatever.— Vid. Plot Nat. Hiji. Stafford, c. 9. §. 88. p. 383.

Dr <?ic-Br idg E, Pons fubdutlarius, is fuch a one as is made faft only at one end, with hinges; fo that the other end may be lifted up ; in which cafe the bridge Hands upright, to hin- der the paflage of a moat, or the like. There are others made to draw back, to hinder the pafTage, and to thruft over again to afford a pafTage. And others, which open in the middle ; half of which turns away to one fide, and the other to the other ; being joined again at plea- fure : but thefe have this inconvenience, that one half of them remains on the enemy's fide.

The Marquis de l'Hopital has given the conftruction of a curve, in which a weight will always be a counter-balance to a draw -bridge ; which the younger Bernoulli has ftiewn to be no other than the cycloid. — Fid. An. Erud. Lipf. an. 1695. p. 56. feq.

jfTy/V/f-BRiDGE, Pont volant, or Pons duSfarius, an appellation given to a bridge made of pontoons, leathern boats, hollow beams, cafks, or the like, laid on a river, and covered with planks for the pafTage of an army.

.F/jwff-BRiDGE, Pont volant, more particularly denotes abridge compofed of one or two boats joined together, by a fort of flooring, and furrounded with a rail or balluftrade ; having alfo one or more mafts, to which is faftened a cable, fup- ported at proper diftances by boats, and extended to an an- chor, to which the other end is faftened, in the middle of the water. By which contrivance, the bridge becomes move- able, like a pendulum, from one fide of the river to the o- ther, without other help than the rudder. — Such bridges fome- times alfo confift of two ftories, for the quicker paflage of a great number of men j or that both infantry and cavalry may pr.fs at the fame time,' — Davil.

Fl/mg or Fhating-BiUDGE} is ordinarily made of two finall 2

bridges, laid one over the other, in fuch manner, as that the uppermoft ftretches and runs out, by the help of certain cords running through pulhes placed along the fides of the under-bridge, which pufh it forwards till the end of it joins the place it is defigned to be fixt on.

When thefe two bridges are ftretched out at their full length, fo that the two middle ends meet, they are not to be above four or five fathom long, becaufe if longer they will break. Their chief ufe is for furprizing out-works, or pofts that have but narrow moats.

In the memoirs of the royal academy of fciences, we find a new contrivance of 2. floating-bridge, which lays itfelf on the other fide of the river. — Fid. Hiji. Acad. R. Scienc. an. 1713.

P' I0 4-

Bridge of communication, is a bridge made over a river ; by which two armies or forts, feparated by the river, have a free communication with one another.

Bridges of boats, are either made of copper, or wooden boats faftened with ftakes, or anchors ; and laid over with planks. See Boat.

One of the moft notable exploits of Julius Csefar, was the expeditious making a bridge of boats over the Rhine : mo- dern armies carry copper boats, called pontoons, to be in rea- dinefs for the making bridges : feveral of thefe being joined fide by fide, till they reach acrofs the river, and planks laid over them, make all plain for the men to march on. See Pontoon.

There are fine bridges of boats at Beaucaire, and Rouen, which rife and fall with the water; yet that at Seville is laid to exceed them both.

The bridge of boats at Rouen, built in lieu of the ftately Hone-bridge erected there by the Romans, is reprefented by a modern writer, as the wonder of the prefent age ; it al- ways floats ; and rifes, and falls with the tide, or as land- waters fill the river ; it is near 300 yards long, and is paved with ftone juft as the ftreetsare; carriages with the greateft burdens go over it with eafe, and men and horfes with fafe- ty, though there are no rails on either hand. The boats are very firm, and well moored with ftrong chains ; and the whole well looked to, and conftantly repaired, though now very old.

D.

DEGREE, in geometry, a divifion of a circle, including a three hundred and fixtieth part thereof. See Circle.

Every circle, great and fmall, is fuppofed to be divided into 360 parts, called degrees: the degree is fubdivided into 60 leffer parts, called minutes: the minute into 60 others, called feconds : the fecond into 60 thirds, &c. — It follows, that the degrees, minutes, £3V. of greater circles, are greater than, thofe of lefs. See Minute, Second, &c. The fubdivifions of degrees are fractions, whofe denomina- tors proceed in a fexagecuple ratio ; that is, a firft minute is = /o> a fecond = ,- 6 V OJ a third = 2 i- 6 ~ - y ¥& But thefe denominators being troublefome, their logarithms are fubftituted in common ufe, as indices thereof. "See Loga- rithm.

Thus, a degree, as being the integer or unite, is denoted by o, a firft minute or prime by 1, a fecond by 2 or ", a third by 3 or "', C3V. Accordingly 3 degrees, 25 minutes, 16 thirds, are wrote, fa 25', 16'". See Sexagesimal. But though the ancient Egyptians, to whom this divifion is ufually afcribed, did, by means hereof, free aftronomical calculations from fractions; fince fexagefimal fractions may be handled as integers ; and were very happy in the choice of fuch a number of degrees in the circle, as admitted of a juft divifion by 2, 3, 4, 5, 6, 8 and 9. Yet Stevinus, Oughtred, Wallis, &c. with good reafon, wifh the fexage- fimal fractions fet afide, and decimals taken in their room. For in decimals there is no occafion for reducing lefler frac- tions into greater, or greater into lefler ; which is a tedious article in fexagefimals. Stevinus even holds, "that this divi- fion of the circle which he contends for, obtained in the wife age, in Seculo Sapienti. Stevin. Cofmog. Lib. 1. Def. 6. See Decimal.

The magnitude or quantity of angles is accounted in degrees. Thus, we fay, an angle of 90 degrees; of 70 degrees, 50 mi- nutes; of 25 degrees, 15 minutes, 45 feconds. See Angle. Such a ftar is mounted fo many degrees above the hori- zon, declines fo many degrees from the equator. See Al-

TITUDE,

Such a town is fituate in fo many degrees of longitude and

latitude. See Longitude and Latitude.

A fign includes 30 degrees of the ecliptic. See Sign.

Degree of latitude, is the fpace of 365184 Englifti feet in- cluded between two parallels of latitude. See Latitude.

Degree of longitude, is the fpace between two meridians; the quantity of which is variable according to the latitude. See Longit ude.

Thefe expreflions are borrowed from the ancients, who were acquainted with a very large extent of earth from eaft to

weft,